FIG. 31 is a block diagram showing the structure of a conventional liquid crystal display apparatus. In FIG. 31, the conventional liquid crystal display apparatus includes a controller 910, a source driver 911, a gate driver 912, and an IPS-type liquid crystal panel 913. The controller 910 performs, as a main role, timing control of the source driver 911 and the gate driver 912, and alternating-current (AC) drive control of the IPS-type liquid crystal panel 913.
The AC drive of the liquid crystal panel is described below. In the liquid crystal panel, a liquid crystal material is used to seal between two electrodes on a pixel basis so as to change the voltage to be applied between these electrodes. Thereby, liquid crystal molecules are aligned differently, the optical property is accordingly changed, and image display is thus performed. Generally, in a TN (Twisted Nematic)-type liquid crystal panel, if a direct-current (DC) is applied thereto as a driving voltage, ions in the liquid crystal material move closer to the electrodes, and as a result, a phenomenon called sticking-image occurs with some display images being stayed. This is the reason why the liquid crystal panel often operates with an AC drive, and is generally driven in AC waveforms in which the polarity alternates in synchronization with a vertical synchronizing signal of the video signal. FIG. 32 is a diagram showing a drive voltage waveform of, under such a conventional AC drive method, a test pattern of still images showing gray, white, and gray in the horizontal direction. The lateral axis is a space axis, that is, the lateral axis indicates pixel positions in the horizontal direction. Because of operation with an AC drive, the polarity of the drive waveform alternates on a frame basis, that is, between an odd-numbered frame and an even-numbered frame. Here, as shown in FIG. 32, the polarity of the drive waveform also alternates between any horizontally adjacent pixels, which is called dot reverse, or column reverse, and is one popular method which is used to reduce flicker often occurring at the time of the AC drive.
On the other hand, as a technique for improving the viewing angle performance of liquid crystal displays, liquid crystal panels of an in-plane switching type (hereinafter, referred to as IPS (in-Plane Switching) type) have been recently developed. FIG. 33 is a set of diagrams showing the structure of such an IPS-type liquid crystal panel. Specifically, FIG. 33(a) is a diagram viewed from a direction perpendicular to its display surface, while FIG. 33(b) shows a section view. As shown in FIG. 33, in the IPS-type liquid crystal panel, two electrodes for driving a liquid crystal, i.e., a common electrode 921 and a drain electrode 922, which is connected to a source line 920 through a pixel transistor 923, are in such a comblike structure as occluding each other on the same surface of a glass substrate 924. In the IPS-type liquid crystal panel, switching takes place with respect to the liquid crystal of a liquid crystal layer 927 by a horizontal electric field generated between these two electrodes 921 and 922, thereby advantageously realizing the property of quite a wide viewing angle. Here, because the liquid crystal is rather slow in response speed, the IPS-type liquid crystal panels are now mainly used as display panels for still images exemplified by monitors of personal computers. With the improvement of the IPS-type liquid panels and their relating technologies, the IPS-type liquid crystal panels are becoming applicable to displaying moving images such as television signals.
The issue here is, if a liquid crystal display apparatus including a conventional IPS-type liquid crystal panel displays moving images such as television signals, a problem surely arises due to liquid crystal being slow in response speed. Furthermore, the inventors have found another problem through their study in that movement of displaying objects, patterns, and others in the moving images result in tail streaks, which causes image degradation of the region to which those objects, patterns and others moved.
With reference to FIGS. 33 to 37, described in detail below is a mechanism why such a new problem of image degradation occurs if moving images are displayed on the IPS-type liquid crystal panel in the conventional liquid crystal display apparatus.
Initially, the electrode structure of the IPS-type liquid crystal panel is described first in comparison with that of the general TN-type liquid crystal panel.
Generally, in the TN-type liquid crystal panel, a planar transparent electrode (ITO) is provided for each glass substrate placed so as to face each other. With such a structure, the planar ITO works as a stopper when an insulator film over the ITO is removed in the manufacturing process, whereby etching can be done with no overetching being caused. In the IPS-type liquid crystal panel, on the other hand, as shown in FIG. 33, pixel electrodes are provided in a comblike structure (e.g., Al, Cr), that is, the common electrode 921 and the drain electrode 922 are placed so as to occlude each other on the same surface of the glass substrate. With such a structure, when insulator films over those pixel electrodes, that is, a gate insulator film 925 and a protection insulator film 926, are removed in the manufacturing process, electrodes such as the common electrode 921 and the drain electrode 922 work as a stopper for their own part, but there is nothing working as a stopper for a part between the common electrode 921 and the drain electrode 922. Thus, without correct control over the etching speed, there is a possibility for overetching. This is the reason why the insulator films over the pixel electrodes are often not removed in the IPS-type liquid crystal panel, that is, the pixel electrodes remain covered by the insulator films. This is one cause of the tail streaks mentioned above.
FIG. 34 is a set of diagrams showing drive voltage waveforms of the conventional liquid crystal display apparatus including the IPS-type liquid crystal panel in the case where a test pattern showing white, gray, and white is moved by two pixels rightward on a frame basis. In these diagrams each corresponding to a frame, the lateral axes indicate pixel positions in the horizontal direction (space axes), the vertical axes indicate the drive voltage, and the frames are arranged longitudinally in order (discrete time). As already described by referring to FIG. 32, with an AC drive, the polarity alternates on a frame basis in the drive voltage waveform, and further, with a column reverse, the polarity alternates on a pixel basis in the horizontal direction in the drive voltage waveform.
Here, in FIG. 34, focusing on a pixel A shown therein, a time-base representation of any change observed in the drive voltage will lead to a diagram shown in FIG. 35(a). As indicated by the thick line in FIG. 35(a), DC components (low frequency components) of the voltage applied to the electrodes become out of balance when the test pattern passes through. In other words, at the time when the test pattern passes through, the DC voltage is applied to the electrodes of the IPS-type liquid crystal panel.
As described in the foregoing, the electrodes of the IPS-type liquid crystal panel are each covered by an insulator film (SiNx), and therefore with a DC voltage being applied to the electrodes as such, polarization occurs in the insulator films. FIG. 36 is a model diagram showing how polarization occurs as a result of DC voltage application, (−) to the common electrode 921, and (+) to the drain electrode 922. As shown in FIG. 36, DC voltage application to the IPS-type liquid crystal panel causes ions in its liquid crystal layer to move, and due to a resultant uneven distribution of ions, polarization occurs both in the liquid crystal layer and the insulator films covering the electrodes. As a result of such polarization, electric field components are generated so that the electric field applied to the liquid crystal layer is thereby cancelled out. Moreover, the electric field components generated as such keep affecting the electric field applied to the liquid crystal for the duration until the polarization level is lowered.
FIG. 35(b) is a diagram showing an electric field applied to the liquid crystal of the focusing pixel A. As indicated by the thick line in FIG. 35(b), due to polarization resulting from an electrode voltage added with DC components, such an electric field component as canceling out the DC components affects an electric field to be applied to the liquid crystal during a pattern display period and thereafter. Here, focusing on the electric field especially after the pattern has passed through, in frames after the pattern has passed through, the voltage to be originally applied to the liquid crystal of the pixel A is the one showing no change in absolute value as shown in FIG. 35(a). However, as shown in FIG. 35(b), actually applied thereto is such a voltage as increasing and decreasing in absolute value on a frame basis. As a result, AC drive becomes out of balance between positive and negative, causing flicker. As described above, under AC drive, the polarity alternates in synchronization with the vertical synchronizing signal. Accordingly, such flicker occurs in half of the frequency components of the vertical synchronizing signal.
Such flicker increases in proportion to the size of the DC component and the time when the DC component was applied. As an example, by first displaying white for a positive frame and black for a negative frame sequentially for two seconds each, and then displaying gray, a flicker resultantly occurs which is visible even to the naked eye. Also, even if the flicker that has occurred is in such a level as not being visible to the naked eye when the line of sight is fixed, the flicker may become visible once the line of sight is changed. This is explainable by the human eyes as being a sensory organ sensitive to the amount of spatial and temporal changes. When the line of sight is fixed, only the amount of temporal change in brightness becomes a sensory stimulation, but when the line of sight is changed, in addition to the amount of temporal change in brightness, the amount of spatial change in brightness also becomes the sensory stimulation. For example, as shown in FIG. 37, in a display screen 914 of the IPS-type liquid crystal panel, if an exemplary test pattern of a white BOX 915 is moved leftward within a gray background 916, the human eyes follow this movement. Since synchronization is established between the movement of the test pattern and flicker, as indicated by the arrows in FIG. 38, the line of sight has a directional property in the temporal and spatial direction, and as a result, flicker occurs as if a pattern of streaks is moving. As a result, a tail echo 917 such as the one shown in FIG. 37 is perceived. As such, unlike general afterglow, the tail echo 917 appears as a pattern of streaks, causing considerable image degradation of the moving images.
Here, as described in the foregoing, one cause of the tail echo 917 is an uneven distribution of ions (liquid crystal polarization) as a result of DC voltage application. This polarization occurs as a result of impurity ions in the liquid crystal panel moving in response to the electric field. Accordingly, the polarization level is increased as the density of such impurity ions is increased in the liquid crystal panel.
Conventionally, in order to increase the response speed of a liquid crystal material used for the IPS system, its viscosity has been on a downward path. Further, in order to lower the drive voltage, Δ∈ (anisotropic dielectric constant) has been on an upward path. Through such a development, the liquid crystal material for the IPS system generally includes a CN liquid crystal, or the liquid crystal material to be used therefor is high in ∈ (permittivity). However, with such a liquid crystal material including a CN liquid crystal or being high in ∈, impurity ions are to be easily captured in the liquid crystal. As a result, as already described, polarization occurs easily so that an electrical charge on the resultant interface is increased.
Moreover, for the purpose of reducing a streaking-image, for example, the liquid crystal panel may be filled with liquid crystal of a low resistance or provided with an orientation film, the liquid crystal panel may be irradiated with a UV ray, or the liquid crystal therein may be mixed with any additive. If these are the cases, however, the ion density in the liquid crystal is resultantly increased so that the above-described echo phenomenon occurs more apparently, considerably degrading the quality of the moving images.
Therefore, an object of the present invention is to provide a liquid crystal display apparatus and method in which no echo phenomenon occurs even if moving images are displayed by using a liquid crystal panel.